Browser Support

Dedicated Worker / SharedWorker: Nearly all browsers

Service Worker: Chrome 40+ · Firefox 44+ · Safari 11.1+ · Edge 79+

Worklet: Chrome 49+ / Safari incomplete support

Overview

Many people confuse Service Worker with Dedicated/Shared Workers, thinking they're all "background threads". Actually, they solve completely different problems:

• Dedicated / Shared Worker: Help main thread share compute load, communicate via postMessage
• Service Worker: Acts as a proxy between browser and network, intercepts/manages requests via event-driven fetch responses

Service Worker is not a Worker thread; it's a separate lifecycle running under the registered domain, more like a script running in the background that can intercept network requests, push notifications, manage caches.

A Table Explains the Differences

Feature	Dedicated Worker	Shared Worker	Service Worker
Creation	new Worker(url)	new SharedWorker(url, name)	navigator.serviceWorker.register(url)
Thread model	Independent thread	Independent thread (shareable across tabs)	Independent thread (event-driven)
DOM access	No	No	No
Network requests	fetch	fetch	fetch + Intercept fetch
Cache management	No	No	Cache API / Cache Storage
Push notifications	No	No	Push API / Notification API
Lifecycle	terminate() / close()	close()	Install → Activate → Deprecated
Offline capability	No	No	Can implement offline apps
Scope	Own	Same origin	Has path scope
Communication	postMessage	postMessage (via port)	Events (fetch, push, message)
Use case	Compute-intensive tasks	Cross-tab shared state	Network proxy, PWA offline, push

When to Use Which?

Do you need to intercept network requests?
  ├── No → Do you need cross-tab shared state?
  │         ├── No → Dedicated Worker
  │         └── Yes → Shared Worker (or BroadcastChannel)
  └── Yes → Service Worker (PWA / offline apps / push)

Notes

• HTTPS required: Service Worker must run under HTTPS (except localhost)
• Scope restrictions: Service Worker's scope determines which path requests it can intercept; subpaths are covered, parent paths are not
• Multiple versions can coexist: Old version is garbage collected after all pages stop using it
• self in Service Worker is ServiceWorkerGlobalScope: Has caches, fetch, clients APIs not available in Dedicated Worker
• Service Worker events disappear after exit: It only lives when events trigger; browser suspends it when idle — this is by design, not a bug